The Long War Journal has a detailed write-up on the attack:
A suicide bomber rammed a car packed with explosives into a Rhino Runner transporting NATO troops to the Kabul Military Training Center Kabul, a US military official told The Long War Journal. The Rhino is an up-armored bus that is used to transport large numbers of troops around the capital.
…”The Taliban first had to build a bomb big enough to penetrate the Rhino’s armor, and then they had to position the suicide bomber to execute the attack,” the official said. “Clearly they have been observing our movements and timed this attack.”
It sounds like this was a frequently used transport route. Any fixed pattern is vulnerable to attack but significant variation in timing and route is not always possible. I’m sure that sort of review is underway now.
Recommended on Twitter: @billroggio. Editor of The Long War Journal & Senior Fellow at The Foundation for Defense of Democracies.